Volcano Watch: Drive along Saddle Road reveals outstanding volcanic geology

Highway 200, the Daniel K. Inouye Highway, crosses Humu‘ula Saddle, which separates Maunakea and Mauna Loa, the two largest volcanoes on the Island of Hawaii. This saddle showcases outstanding volcanic geology and is easy to reach for “roadside geologists.”

For the first 19 miles (30.6 km), Highway 200 ascends almost a mile (1.6 km) above sea level through mostly young native forest. The road crosses lava flows and patches of ashy soil ranging in age from 150-4,000 years old. Rainfall is so abundant here — up to nearly 300 inches (760 cm) per year — that vegetation covers most outcrops.

At about Milepost 19, Highway 200 emerges onto young looking, lightly vegetated ‘a‘a lava from the 1855-56 Mauna Loa eruption. This flow is one of seven that traveled from Mauna Loa toward Hilo in recorded history.

The 1855-56 eruption lasted a year and a half. Imagine the commuter’s nightmare if this were to occur today.

Between mileposts 19 and 20, tall trees occupy numerous kipuka growing on Punahoa pahoehoe lava erupted from Mauna Loa 3,100 to 3,200 years ago. Near milepost 21, the road drops into a kipuka floored by the 400-year-old Mauna Loa ‘Ainahou ‘a‘a flow.

Near Saddlehouse Road, the road rises onto Mauna Loa ‘a‘a lava erupted in1935-36. This flow was the first-ever in the United States and its territories that authorities tried to “manipulate” to protect property downslope. The U.S. Army Air Corps, advised by Thomas Jaggar, director of the Hawaiian Volcano Observatory at the time, dropped 600-pound (272 kg) bombs on the active flow about 10 miles (16 km) southwest of the intersection. The goal was to disrupt the lava channel, causing the flow to spread laterally rather than continue downhill. The impact was negligible.

On the right side of the highway just after Saddlehouse Road, the 1935-36 lava is rumpled into a spectacular series of low compressional ridges (“lava ogives”). These features are easiest to spot in early morning or dusk light. Each ridge indicates you are near the end of the flow, where the lava became too stiff to continue. The actual terminus is about 2 miles (3.2 km) away.

South of the road between mileposts 22 and 23, a pullout marks the start of the Pu‘u ‘O‘o Trail, which crosses a landscape of diverse kipuka and younger flows. Spectacular old-growth ohia and tree ferns appear in scattered groves. The trail, which used to continue all the way to Volcano ranchlands, now ends after 4 miles (6.4 km) at the edge of the 1984 flow, the youngest lava to head toward Hilo.

Past milepost 22, road builders deliberately constructed Highway 200 atop the 1935-36 ‘a‘a flow. That’s because rubbly ‘a‘a is much easier than solid pahoehoe to grade.

Near milepost 23, Highway 200 approaches the base of Maunakea. The lumpy, grassy landscape to the right of the road results from the eruption of hawaiite, a unique type of lava that is notably stiffer and richer in silica and potassium than Mauna Loa flows. The numerous upslope cones belong to the Pu‘uloa volcanic field, which last erupted about 4,600 years ago and was the source of the hawaiite. Pu‘uloa vents extend to the top of Maunakea.

A well-loved local landmark, Pu‘uhuluhulu, is at the crest of the Saddle. This densely forested Maunakea cinder cone is entirely surrounded by younger Mauna Loa flows. An abandoned quarry at its eastern end exposes the cone’s interior, showing that sometime after eruption of loose, porous cinders, narrow dikes of basaltic lava worked their way into the edifice. The dike rock matches the chemistry of Mauna Loa’s lavas, suggesting they intruded much later. But how and why they did remain a head-scratcher for geologists.

Near the northwestern foot of Pu‘uhuluhulu is a stone wall, built by hand in the late 19th century by cattle ranchers. The 1935-36 lava lapped around and overtopped the wall in places. Stunning inflation features illustrate that the flow inflated like rising bread crust on both sides of the wall after initially coming to rest against it.

Other interesting features might be discovered in the Humu‘ula Saddle, such as ancient dune fields, glacial deposits, and rocks from Earth’s deep interior. We’ve described only a few here, hopefully instilling a new appreciation for our spectacular ‘aina.

Volcano activity updates

This past week, Kilauea Volcano’s summit lava lake level fluctuated with summit inflation and deflation, ranging about 31-48 m (102-157 ft) below the vent rim. On the East Rift Zone, the 61g lava flow remained active downslope of Pu‘u ‘O‘o, with scattered breakouts on the coastal plain. The 61g flows do not pose an immediate threat to nearby communities.

Mauna Loa is not erupting. A few small-magnitude earthquakes occurred beneath the summit caldera and upper Southwest Rift Zone, mostly at depths less than 5 km (3 mi), with a few more on the volcano’s west flank at depths of 5-13 km (3-8 mi). GPS measurements continue to show deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone. No significant changes in volcanic gas emissions were measured.

No earthquakes were reported felt on the Island of Hawaii during the past week.

Visit the HVO website (https://volcanoes.usgs.gov/hvo) for past Volcano Watch articles, volcano updates and photos, recent earthquake info, and more. Call for summary updates at 808-967-8862 (Kilauea) or 808-967-8866 (Mauna Loa). Email questions to askHVO@usgs.gov.

Volcano Watch (https://volcanoes.usgs.gov/hvo/hvo_volcano_watch.html) is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.